But a report in October is said to have warned about a possible "sudden and catastrophic collapse".

Transport for London has told us the rate at which the cable strands were breaking started to increase significantly in summer 2011.

Garrett Emmerson, Chief Operating Officer for Surface Transport at TfL said: "The Hammersmith flyover was built in 1961 from concrete sections. Unusually, it is supported by 30 miles of tensioned steel cables running along its length and enclosed in concrete, making them impossible to inspect. Since TfL began managing the flyover in 2000, it has monitored its integrity and strength.

"From 2009, TfL began looking in more detail using acoustic listening techniques to detect any breaks in the individual strands making up the cables. We knew the flyover would need work to strengthen it, but the assessment at the time suggested not for another ten years.

"Last summer, the rate at which these cable strands were breaking started to increase significantly. Therefore, TfL began opening up parts of the concrete structure to look at their physical condition. In several areas, the cables were in a much poorer state than had previously been understood.

"In the week prior to Christmas 2011, following further deterioration to the cables, we took the decision to keep the flyover closed to carry out more detailed assessment of the complete structure. On 13 January 2012, TfL reopened the flyover to light traffic in one lane in each direction, whilst work continued to strengthen key sections of the flyover. A team of engineers worked 24 hours a day, seven days a week to complete the work on top and within the flyover, often in a crawl space far less than human height, and reopened it to all traffic in January."